do i need to do a motion to modify to get my child support dropped from when i was in prison

In many states in order to get your child support or spousal support modified you usually have to file a motion based on "changed circumstances." This shouldn't be a problem, since you were in prison.  But you must do something about notifying the court of your situation, otherwise the support payments will add up with interest and it could also involve a criminal contempt.  If you inform the court and seek a modification, this will eliminate any "willfulness" on your part.

I hope this information is helpful to you.